Aerospace Composites

Engineering plastics bring the possibility of property enhancement through fibre reinforcement. Such are aerospace composites. Reinforced plastics, thermoplastics and thermosets, can benefit from fibre reinforcement. Both are very different. Thermosetting plastics, which are brittle, use fibre reinforcement in mouldings. Thermoplastics has only developed structural plastics more recently. Glass fibres are the main form of reinforcement used for plastics because they offer a good combination of strength, stiffness and affordability. Improved strengths and stiffnesses can be achieved with other fibres such as aramid Kevlar, or carbon fibres, but these are expensive. The latest developments also include the use of hybrid composite systems to get a good balance of properties at an acceptable sheet piling cost.

 

Aircraft composites in the boeing 787. Issues with composites.

The real improvement in performance offered by the Boeing 787 may have more to do with the new engines than the composite aircraft structures. In fact General Electric (Rolls-Royce is the other engine choice) is offering 15% less fuel burn than on previous wide-body engines. The 50% composite content and in particular the composite fuselage of the 787 presents enormous challenges to Boeing engineers and manufacturing teams. In fact Boeing engineers did not want to go down the composite route. Manufacturing large composite structures is difficult, as the process is complex and dependent on high skill levels amongst employees. In order to produce the huge fuselage barrels, composite tape, woven from ultra-strong polymer fibres, is soaked in liquefied polymers and then baked in an autoclave. The process requires a craft-like skill level and is hard to automate. Ensuring consistent quality is difficult, as Boeing has already found on the first few fuselage barrels it fabricated.

The disadvantage of composites for mass production is their total unsuitability to high-volume production rates. All current aviation composite systems are “thermoset” systems. That means that the material must be “cured” at high temperatures and pressures. For high-volume/low-cost work, the auto industry uses a “thermoplastic” system that cures rapidly in moulds. This process is basically like injection moulding of plastics. The FAA has never certified such a system for aviation usage. Furthermore, the cost of developing and certifying a thermoset system would require a major investment as well as add significant risk to the certification process. So while composites have both an appropriate application in some areas of aviation (low-volume production and/or nonstructural parts) and a very bright future in aviation, aluminum is probably the best choice for a high-volume jet such as the Eclipse 500.

Damage Detection

Boeing is claiming that the 787 will need less maintenance checks than a metallic aircraft. It has been suggested that the first major structural examination of the aircraft might not be necessary for 12 years, because of less concern over corrosion. But this view neglects a key fact about composites. Damage to metallic structures is easy to see, although assessing the effects of the damage is often complex.

Much is known about fracture mechanics and the impact of loads and stress on cracks in metals. Less is known about composite. But the bigger problem is that damage to composite materials is more difficult to detect and interpret. Impact damage may not be visible and moreover, the real structural harm to composite material after an impact may be physically removed from the actual impact point.

With a composite fuselage there are certain to be bumps and shunts on the ground and ramp from service vehicles causing varying degrees of damage to the structure. Because of this, operators of the 787 will need to implement a vigilant regime of damage inspection and investigation. The evaluation of complex composite structures often requires labour-intensive, expensive methods because of multiple failure modes, difficulty detecting damage, and the large scale of the aircraft structures. To get around this, the only course of action is investment in expensive ultra sound and laser doppler test equipment.

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